This invention relates to a cathode ray tube, and more particularly, to a magnetic focusing type cathode ray tube.
A prior art cathode ray tube includes an electrostatic focusing type of cathode ray tube and a magnetic focusing type of cathode ray tube. But, it is only the former type of cathode ray tube that is already being put to practical use. As compared with this former type, however, the latter type of cathode ray tube has a high resolution, and necessitates no focusing voltage with the result that its power source circuit is simplified and the problems of withstanding voltage are lessened. For this reason, the reliability is enhanced with respect to this latter type, and the manufacturing cost thereof is decreased at the same time. Under these circumstances, developments continue for practical applications of the magnetic focusing type cathode ray tube.
Meanwhile, the prior art magnetic focusing type cathode ray tube uses an electron gun of magnetic focusing lens system, which is comprised of a cathode member and a focusing magnetic yoke assembly. An in-line type electron gun has three cathodes arranged in an in-line form and a pair of magnetic yokes each having electron beam passing holes corresponding to those cathodes, respectively. The magnetic yokes are disposed in opposed relationship to each other and are coupled by a pair of permanent magnets. For example, the permanent magnets are positioned above and below a central electron beam path, respectively, so that they have their N pole on the cathode side and their S pole on the screen side. Further, each magnetic yoke is provided with cylindrical magnetic elements protruding from the periphery of its electron beam passing holes.
In the above-mentioned electron gun, the magnetic force lines generated from the N pole of the permanent magnets are absorbed into the cylindrical magnetic elements of the magnetic yoke of the S pole side from those of the magnetic yoke of the N pole side, and thus are returned to the S pole of the permanent magnets. At this time, focusing magnetic fields are formed in the magnetic gaps between the cylindrical magnetic elements of the magnetic yokes placed in opposed relationship to each other. That is, a focusing magnetic field is formed in each of the three electron beam paths with a result that the electron beams from the cathodes are focused by the action of the focusing magnetic fields thus formed. Ideally, a complete magnetic focusing of the electron beams is obtained solely by the action of the magnetic fields of the permanent magnets alone. Actually, however, there is a magnetic field directed from the N pole side, i.e., the cathode side yoke to the cathode, and from the screen to the S pole side, i.e., to the screen side yoke. These external magnetic fields have a deflecting effect upon the side electron beams, which are thus deflected vertically. As a result, when the three electron beams are converged by a 4-pole magnet of ring shape mounted outside of the neck portion, the resultant beam spot has an elliptical shape, and bring about to lower a focusing precision.